Current-limiting fuse

ABSTRACT

A fuse comprising a generally tubular, electrically insulating casing having a pair of terminal members or ferrules disposed adjacent to the opposite ends of said casing. One or more fusible elements is disposed in the casing and connected between the terminal members. An additional fuse wire or restraining wire is disposed in the casing and connected between said terminal members with means provided adjacent to one end of the casing which responds to the melting of the fusible element or elements to change position. A pair of electrically conducting ring members are disposed at the opposite ends of the casing to support a plurality of fusible elements, where provided, in radially and circumferentially spaced positions inside the casing.

United States Patent Cameron [4 1 Jan. 18, 1972 [54] CURRENT-LIMITING FUSE [72] Inventor: Frank L. Cameron, Irwin, Pa. I js'figrgzz [73] Assignee: Westinghouse Electric Corporation, Pitt- AttorneyA. T. Stratton and Clement L. Mcl-lale sburgh, Pa.

22 Filed: Dec. 31, 1969 [571 ABSTRACT [21] AppL No; 889,598 A fuse comprising a generally tubular, electrically insulating 1 casing having a pair of terminal members or ferrules disposed adjacent to the opposite ends of said casing, One or more fusi- U-S- ..337/244, ble elemgnts is disposed in casing and connected between [5 1] Int. Cl ..H0lh 85/12, HOlh 85/30 the terminal mmbers An additional fuse wire or restraining [58] Field of Search ..337/7, 148, 158, 159, 161, wire is disposed in the casing and connected beween said 4" 2 232344 293 minal members with means provided adjacent to one end of i the casing which responds to the melting of the fusible ele- [56] References Cited ment or elements to change position. A pair of electrically UNITED STATES PATENTS conducting ring members are disposed at the opposite ends of the casing to support a plurality of fusible elements, where geenan et gf; provided, in radially and circumferentially spaced positions inameron n... side the casin 2,866,037 12/1958 Stewart ..337/l6l g 2,797,279 6/l957 Brandt et a1. 337/244 10 Claims, 4 Drawing Figures |o4A I03 (I048 /I 'l/ 1 cunasNr-LmrrrNc FUSE BACKGROUND OF THE INVENTION In the construction of certain types of fuses particularly those of the totally enclosed, current limiting type, it is desirable or necessary to provide an externally visible means for indicating that the fuse has blown or operated to interrupt an abnormal or fault current. One general type of fuse construction includes an indicating means which comprises a spring biased plunger which is normally restrained from movement by one or more fusible wires or restraining wires disposed inside the associated casing such as disclosed in U.S. Pat. Nos. 2,306,153; 2,320,494, 2,417,268,- 2,435,844, 3,134,874 and 3,179,774. It has been found that the indicating means of the various fuse constructions disclosed in the above patents have certain disadvantages, particularly when applied at relatively higher voltages, such as 2,400 volts and higher voltages. For example, if a gaseous discharge from a fuse of the type described'is permittedby the indicating means whichfonns part of the fuse during an interrupting operation of the overall fuse, a flashover may occur between adjacent fuses particuthe above type, particularly at relatively high interrupting and continuous current ratings, is to properly position and support a plurality of fusible elements within the associated casing in such a way as to facilitate and to promote the formation of fulgurite'around each fusible element in cooperation with the associated arcquenching material in which the fusible elements are normally embedded.

It is therefore desirable to provide an improved current limiting fuse construction which overcomes the above-mentioned problems and disadvantages.

SUMMARY OF THE INVENTION 'Inaccordance with the invention, a current limiting fuse includes a generally tubular, electrically insulating casing having a pair of terminal members or ferrules-disposed adjacent to the opposite ends of said casing. One or more fusible elements is disposed in the casingand connected between the terminal members ferrules.- One of theterminal members or ferrules includes an openingat one end of the casing. An indicating meansis disposed inside the casing at the latter end of said casing and includes a generally tubular, electrically insulating housing'with one end of said housing extending around the opening in the adjacentterminal member. A plunger is normally disposed in said housing and includes a flange portion axially intermediate the ends of said plunger and an outer portion on one side of said flange portion. A spring is also disposed in said housing with one end of said spring bearing against the flange portion of the plunger to bias the plunger outwardly from the associated housing. In order to normally restrain the plunger from outward movement under the influence of the associated biasing spring, a restraining wire is connected between the plunger and the terminal member or ferrule at the other end of the casing. In order to limit the outward movement of the plunger, a means is disposed around the opening in the terminal member or ferrule which may comprise a separate tubular member which projects axially inthe latter means and said flange portion during the operation of the overall fuse.

In another aspect of the invention, a pair of electrically conducting ring members may be disposed at the opposite ends of the associated casing to position and support a plurality of fusible elements in first and second radially spaced groups of circumferentially spaced fusible elements. Each ring member includes a central opening and a plurality of inwardly projecting portions angularly spaced to define first and second radially spaced groups of support surfaces around said opening, the corresponding support surfaces of the respective ring members being substantially aligned with each other. Where the fusible elements are formed from r'ibbonlike, electrically conducting material, the support surfaces may be recessed in radially extending grooves each having a width adapted to receive one of the associated fusible elements. The fusible elements are assembled with the opposite ends extending over the outer peripheries of the associated ring members and the ends of the associated casing and are secured between the associated terminal members'or ferrules and the adjacent ends of said casing.

It is therefore an object of this invention to provide a new and improved current limitingfuse structure.

BRIEF DESCRIPTION 'OF THE DRAWINGS Other objects of the invention will be apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a side elevational view of a double-barrelled fuse structure embodying the teachings of the invention;

FIG. 2 is an enlarged longitudinal view, in section, of one of the fuse units which forms part of the fuse structure shown in FIG. 1;

FIG. 3 is across-sectional view of the fuse .unit shown in FIG. 2, taken along the line IIl-III in FIG. 2; and

FIG. 4 is a fragmentary view of a portion of the fuse structure shown in FIG. 2, illustrating a modification of the indicating means which forms part of the fuse structure shown in FIG. 2.

DESCRIFTIONOF THE PREFERRED EMBODIMENTS Referring now to the drawings and FIG. 1 in particular, there is shown a doubIe-barrelled fuse assembly 10 which includes a plurality of individual fuse units 20 which, as shown, are electrically connected in parallel by the tenninal end plates or members 102 and 104 which are disposed at the opposite ends of the fuse assembly-l0.

As best shown in FIGS. 2 and 3, each of the fuse units 20 is of the current-limiting type and is particularly adapted for application at relatively higher voltages, such as 2,400 volts and higher. As shown in FIG. 2, each of the fuse units 20 includes a generally tubular casing or housing 30 which is formed from a suitable electrically insulating material which has sufficient structural strength to withstand the thermal conditions and internal gas pressures which may result during the operation of the fuse unit 20, such as a glass-reinforced melamine resin material. In order to close off the opposite ends of the casing 30 of each of the fuse units 20 and to provide means for making electrical connections to the fuse units 20 adjacent to the opposite ends thereof, a pair of generally tubular terminal members 82 and 84 are secured to the opposite ends of the casing 30 of each fuse unit 20 by suitable means, such as the magnetic fonning method which is described in detail in US. Pat. No. 3,333,336 which issued Aug. 1, 1967 to F. L. Cameron and W. C. Good and which is assigned to the same assignee'as the present application. In addition, each of the tenninal members 82 and 84 may be also secured to the associated casing 30 by a suitable bonding or adhesive material, such as an epoxy cement, as indicated at 75 in FIG. 2. Each of the tenninal members 82 and 84 is formed from an electrically conducting material, such as copper, whose outer surfaces may be silver plated and the terminal members 82 and 84 are secured, in turn, to the adjacent terminal end plates 102 and 104, respectively by suitable means, such as brazing or welding. It is to be noted that the terminal members 82 and 84 together with the adjacent portions 1023 and 1048, respectively, of the terminal end plates 102 and 104, respectively, form a pair of terminal end caps or ferrules which are disposed at the opposite ends of the casing 30 and which telescope axially over the opposite ends of the casing 30 of each fuse unit 20.

In order to assist in supporting the fusible elements 52 and 62, which will be described in detail hereinafter, and to properly position the fusible elements 52 and 62 inside the casing 30 of each fuse unit 20 at different locations which are radially spaced from the inner bore of the casing 30, each of the fuse units 20 includes a pair of electrically conducting terminal ring members 72 and 74 which are disposed to bear against the opposite ends of the casing 30 of each fuse unit inside the associated terminal members 82 and 84, respectively. The vring members 72 and 74 are identical in structure and only the ring member 74 will be described in detail. As best shown in FIG. 3, the ring member 74 includes a relatively large central opening as indicated at 83 and includes a plurality of portions 85 which are generally triangular in configuration and which project radially inwardly from the outer periphery of the ring member 74 around the opening 83. More specifically, as shown in FIG. 3, the projecting portions 85 are circumferentially or angularly spaced from one another around the central axis of the ring member 74 by angles of approximately 90 in a symmetrical arrangement. The projecting portions 85 each includes at its inner end a radially extending recess or groove as indicated at 87 to provide a radially inner group of supporting surfaces at the bottoms of the respective recesses in said ring member 74 which are circumferentially spaced from one another. The ring member 74 also includes a plurality of pairs of radially outer grooves or recesses 89 which are disposed on the opposite sides of each projecting portion 85 at the outer periphery of the opening 83 to provide a plurality of support surfaces for the associated fusible elements 62 in a radially outer group of support surfaces. The bottom surfaces of the recesses 89 are the support surfaces for the associated fusible elements 62 as will be described in greater detail hereinafter. It is to be noted that the support surfaces provided in the grooves or recesses 89 are also circumferentially spaced from one another around the periphery of the opening 83 as shown in FIG. 3. It is also important to note that the corresponding support surfaces in the ring member 74 as viewed in FIG. 2 and the lower ring member 72 are substantially aligned with one another adjacent to the ends of the respective fusible elements 52 and 62.

In order to normally provide an electrically conducting path between the terminal members 82 and 84 and the associated terminal end plates 102 and 104 respectively, first and second groups of fusible elements or links 52 and 62, respectively, are

disposed inside the casing 30 of each fuse unit 20 to extend axially between and to electrically connect the terminal members 82 and 84, as best shown in FIGS. 2 and 3. As described previously, each of the terminal ring members 72 and 74 includes first and second generally concentric radially spaced groups of angularly or circumferentially spaced recesses or slots 87 and 89, respectively, which are adapted to receive the ends of the first and second groups of fusible elements 52 and 62, respectively. The opposite ends of each of the fusible elements 52 and 62 are assembled into the recesses or slots 87 and 89 provided in the associated terminal ring members 72 and 74 and are secured to the associated terminal members 82 and 84, respectively, by passing or bending the ends of each of the fusible elements 52 and 62 over the respective terminal ring members 72 and 74 and disposing the ends of the fusible elements 52 and 62 between the casing 30 and the respective terminal members 82 and 84 prior to the time that the terminal members 82 and 84 are secured to the casing 30. After the fusible elements 52 and 62 are assembled inside the casing and 84 are secured to the opposite ends of the casing 30 in the manner previously described, the opposite ends of each of the fusible elements 52 and 62 are firmly secured between the ends of the casing 30 and the associated terminal members 82 and 84 with the respective fusible elements being radially and circumferentially positioned by the associated terminal rings 72 and 74, as shown in FIGS. 2 and 3.

As illustrated, each of the fusible elements 52 and 62 is ribbonlike in shape and is composed of a suitable fusible electrically conducting material, such as silver. The recesses 87 and 89 are of a width adapted to receive the ribbonlike material from which the fusible elements 52 and 62 are formed. As described in greater detail in my U.S. Pat. No. 3,134,874 which issued to me May 26, 1964 and which is assigned to the same assignee as the present application, each of the fusible elements 52 and 62 includes a plurality of axially spaced points of reduced cross-sectional area which may be fonned by V-notching the ribbonlike material from which each of the fusible elements 52 and 62 is formed on both sides at spaced points along its length. This construction of each fusible element 52 and 62 results in a series of restricted areas or neck portions which fuse during an interrupting operation of each of the fuse units 20 to provide a series of spaced arcs, the sum of the voltages across said arcs resulting in a relatively high total arc voltage which is particularly desirable in the operation of a current limiting type of fuse. It is also important to note that in order to distribute and limit the mechanical an thermal stresses on each of the fusible elements 52 and 62 which would otherwise result during long periods of service due to changing load conditions to which each fuse unit 20 might be subjected, each of the fusible elements 52 and 62 may be prebent at a plurality of axially spaced points along its length which results in a zigzag configuration which is also described in greater detail in U.S. Pat. No. 3,134,874 previously mentioned.

In order to assist in arc extinction during an interrupting operation of each of the fuse units 20, the balance of the space inside the casing 30 except for the insulating-housing 110 which forms part of the indicating means 50 which will be described hereinafter may be substantially filled with a finely divided or granular or pulverulent arc quenching material or filler as indicated at 56, such as a high purity silica sand or quartz. Other arc. quenching fillers, such as calcium carbonate, may be combined with silica sand in a particular application as described in detail in U.S. Pat. No. 3,213,242 which issued to me Oct. 19, 1965 and which is assigned to the same assignee as the present application. It is to be noted that the terminal end plate 104 includes a removable pipe type plug 103 adjacent to one end of each of the fuse units 20 which is disposed in an opening 104A provided in said end plate as shown in FIG. 2 to permit the filling of the casing 30 with the arc quenching material 56 after the other parts of each fuse unit have been assembled in the manner previously described. It is also to be noted that the axial space between each of the terminal ring members 72 and 74 and the adjacent terminal end plates 102 and 104, respectively, is also filled with the arc quenching material 56, as shown in FIG. 2.

In order to provide an indication of an interrupting operation of each of the fuse units 20 which is visible externally of the fuse assembly 10, an indicating means 50 is disposed adjacent to the lower end of the casing 30 of each of the fuse units 20, as shown in FIG. 2. The indicating means 50 comprises a generally tubular, electrically insulating housing 110 which may be formed from a suitable electrically insulating material such as a polyamide or nylon resin material, such as that sold under the trademark Zytel 101." The lower endof the housing 110, as viewed in FIG. 2, extends around the opening 102A which is provided in the terrnina] end plate 102. As shown in FIG. 2, the lower end of the housing 110 bears against the inside of the terminal end plate 102 and the housing 110 extends axially away from the terminal end plate 102 and passes through the central opening 83 in the terminal ring 30 in the manner just described and the terminal members 82 member 72 at the lower end of the casing 30. In order to securethe housing member 110 to the terminal end plate 102 during the assembly of the fuse unit 20, a generally tubular member 142 may be disposed to pass through the opening 102A in the terminal end plate 102 and to extend axially into the lower end of the opening in the housing member 110 as viewed in FIG. 2. The generally tubular member 142 may be formed from electricallyinsulating material which is the same as that from which the housing 110 is formed and may include a flange portion 1428- which is seated or disposed to bear against a shoulder portion provided around the opening in the terminal end plate 102. In order to secure the housing 110 in the assembled position shown in FIG. 2 suitable means, such as a bonding material which may be of the epoxy resin type, may be disposed between the portion of the tubular member 142 which projects into the lower end of the housing 110 and the interior of the housing 110.

The indicating means 50 also includes a generally cylindrical indicating or actuating plunger 120 which is normally disposed in the inner bore of the housing 110 as shown in FIG. 2. The plunger 120 includes a flange portion 124 axiallyintermediate the ends of the plunger 120 and an outer portion 126 which is disposed on one side of the flange portion 124 and extends or projects axially inside the opening in the tubular member 142, as shown inFIG. 2. The plunger 120 also includes an inner portion 122 which is'disposed on the other side of the flange portion 124 and which is relatively smaller in cross section or size than the outer portion 126.

The indicating means 50 also includes a helically coiled compression spring 130 which is disposed inside the housing 110 to bias the plunger 120 outwardly from the housing 110. The housing 110 includes a transverse or shoulder portion at its inner end which serves as a spring seat for one end of the spring 130 and which includes a central opening, as shown'in FIG. 2. The flange portion 124 of the plunger 120 serves as a spring seat at the other end of the compression spring 130. It is to be noted that one end of the tubular member 142, as indicated at 142A, acts as a shoulder or stop surface which limits the outward movement of the plunger 120 under the influence of the bias spring 130 since the stop surface 142A lies in the path of movement of the flange portion 124 of the plunger 120. l

In order to normally restrain the outwardmovement of the plunger 120 under the influence of the biasing spring 130, an

auxiliary fuse element or restraining wire 42 is electrically and structurally connected between the plunger 120 and the terminal ring member 74 at the other end of the casing 30 through the generally Z-shaped member 86. The restraining wire 42 is formed from a fusible material, such as coppernickel, having a relatively higher electrical resistance than that of the fusible elements 52 and 62. The lower end of the restraining wire 42, as viewed in FIG. 2, is secured to the terminal end plate 102 or the lower terminal member 82 by suitable means, such as brazing or soldering. The restraining wire 42 then passes through the central opening 83 in the lower terminal ring member 72 and into the housing 110 through the central opening provided at the inner end of said housing. The

restraining wire 42 then passes through a transverse opening in the inner portion 122 of the plunger 120 and out of the housing 110 through the central opening at the inner end of said housing. The restraining wire 42 then extends axially through the casing 30 at a central location which is radially spaced from the fusible elements 52 and 62 and is tied or secured to the midportion of the generally Z-shaped member 86 at the upper end of the fuse unit 20. The generally Z- shaped member 86 is formed from an electrically conducting material having excellent spring characteristics, such as phosphorus-bronze wire. As best shown in FIG. 3, the generally Z-shaped member 86 extends transversely or diametrically across the terminal ring member 74 with the transverse outer portions of the generally Z-shaped member 86 hearing against the inside surfaces of the upper terminal member 84 adjacent to the terminal ring member 74. When the generally Z-shaped member 86 is assembled inside the ter-- minal member 84, as shown in FIGS. 2 and 3, the generally Z- shaped member 86 is deformed or charged, considered as a spring, to insure good electrical contact between the ends of the generally Z-shaped member 86 and the terminal member 84. The generally Z-shaped member 86 therefore serves to both support the upper end of the restraining wire 42 during the assembly of each fuse unit 20 and also provides an electrically conducting path between the upper end of the restraining wire 42 and the terminal member 84.

It is to be noted that after the restraining wire 42 is assembled inside the casing 30 and to extend between the terminal members 82 and 84 as just described, an end plug member 131 which may be formed from the same electrically insulating material as the housing 110 may be assembled inside the central openingat the inner end of the housing 110 between the difi'erent portions of the restraining wire 42 to substantially prevent the entrance of the arc quenching material 56 into the housing 110 of the indicating means 50.

In order to cushion the impact of the flange portion 124 of the plunger 120 against the stop surface 142A provided on the tubular member 142 during the operation of the indicating means 50 and to also provide a gas or hermetic seal to close off any opening or clearance that might otherwise exist between the flange portion 124 of the plunger 120 and the inner bore of the housing 1 10, a gasket member 140 is slidably or rollably disposed-on the outer portion 126 of the plunger 120 between the flange portion 124 of the plunger 120 and the stop surface 142A provided on the tubular member 142. The gasket member 140 is preferably of the O-ring type and is formed from a resilient or elastomeric material, such as a Buna-N synthetic rubber or the like.

In the overall operation of the fuse unit 20 it is to be noted that normally the restraining wire 42 carries a negligible or relatively small amount of current due to the relatively higher electrical resistance of the restraining wire 42 compared with that of the fusible elements 52 and 62. Upon the occurrence of apredetermined overload current flowing through the various fusible elements which make up the fuse unit 20, the electrical current through the main fusible elements 52 and 62 will cause each of the fusible elements 52 and 62 to fuse or melt and the electrical current which is normally carried by the fusible elements 52 and 62 will then temporarily transfer to the restraining wire 42 which is electrically connected in parallel with the fusible elements 52 and 62 between the terminal members 82 and 84. The electrical current which is transferred to the restraining wire 42 will produce significant heating of the fuse wire 42 due to the relatively high electrical resistance of the restraining wire 42. The restraining wire 42 will then melt and vaporize the plunger 120 of the indicating means 50 will be released to move axially outwardly from the position shown in FIG. 2 under the influence of the biasing spring 130. As the flange portion 124 of the plunger moves towards the stop surface 142A of the tubular member 142, the gasket member 140 will be actuated to move downwardly, as viewed in FIG. 2, until the gasket member 140 is compressed between the flange portion 124 of the plunger 120 and the stop surface 142A of the tubular member 142. The gasket member 140 will absorb at least a portion of the energy transmitted to the plunger 120 by the spring to thereby prevent the plunger 120 from being broken up against the tubular member 142 and also will form a gas seal around the plunger 120 to prevent the escape or discharge of gases from the fuse unit 20 which might otherwise cause flashovers between the adjacent fuse units 20 which make up the fuse assembly 10 or other nearby fuse units. As indicated previously, the gasket member will move slidably or rollably on the outer portion 126 of the plunger 120 during the operation of the indicating means 50.

After the restraining wire.42 melts or ruptures as just indicated, the electrical current which is momentarily or temporarily carried by the restraining wire 42 will be transferred or returned to the fusible elements 52 and 62. As explained in detail in U.S. Pat. No. 3,134,874 previously mentioned, the

fusible elements 52 and 62 will melt and vaporize at the axially spaced portions of reduced cross section to form a plurality or series of arc voltages along each of the fusible elements 52 and 62 and also to form a glasslike fulgurite along each of the fusible elements 52 and 62. It has beenfound that the radial and circumferential spacing of the different groups of fuse links 52 and 62 which results from the configuration of the terminal ring members or spacers 72 and 74 which are disposed at the opposite ends of the casing 30 facilitates and promotes the desired fon'nation of the fulgurite around each of the fusible elements 52 and 62 due to the transfer of energy from said fusible elements to the arc quenching filler or material 56 in which the various fusible elements are disposed or embedded. In other words, the disclosed construction of each of the fuse units 20 more effectively utilizes the energy absorbing ability of the arc quenching material 56 and more effectively uses said are quenching material to aid in arc extinction because of the desirable positioning of the fusible elements 52 and 62 within the casing 30 which is determined by the construction of the terminal ring members 72 and 74.

- Referring now to FIG. 4 there is illustrated a modification of the structure of each fuse unit 20 in which the overall fuse structure is the same as previously described except that in the indicating means 50' the tubular member 142 is omitted and the means for limiting the outward movement of the plunger 120 comprises a turned-in portion 202A of the terminal end plate 202. As shown in FIG. 4, the terminal end plate 202 provides a stop surface 2028 by means of the tumed-in portion 202A which is disposed around the opening in the housing 110 in which the plunger l20 is disposed.

In the operation of the modified form of the invention shown in FIG. 4, the gasket member 140 is disposed between the flange 124 on the plunger 120 and the stop surface 2028 on the terminal end plate 202 to be compressed during the operation of the indicating means 50' to thereby cushion the impact of the plunger against the stop surface 2028 and to move slidably or rollably on the outer portion 126 of the plunger 120 to provide a gas-seal around any clearance or opening that might otherwise exist between the plunger 120 and the inner bore of the housing 1 10 to prevent the escape of gaseous products during the operation of the overall fuse unit in which the indicating means 50 is incorporated.

It is to be understood that the teachings of the applicants invention may be applied to fuse structures employing the terminal ring members in an overall fuse structure as disclosed which may incorporate other types of indicating means than the indicating means 50. It will also be understood that the teachings of the applicants invention with respect to the improved indicating means 50 may be employed in fuse structures of other types than the current limiting fuse structure which is specifically described.

The apparatus embodying the teachings of this invention has several advantages. For example, an improved indicating means is disclosed for fuse structures which substantially reduces or eliminates the possibility that the spring-biased plunger which forms part of the indicating means may break up during the operation of the indicating means and also prevents gaseous discharges through such an indicating means from the totally enclosed fuse structure which might cause flashovers between adjacent fuse units. In addition, an improved means is disclosed for assisting in supporting and positioning a plurality of fusible elements in radially spaced groups of circumferentially spaced fusible elements. Finally, the disclosed current limiting fuse structure more effectively utilizes the energy absorbing ability of an arc quenching material in fuse structuresof the disclosed type.

Since numerous changes may be made in the above described apparatus and different embodiments of the invention may be made without departing from the spirit and scope thereof, it is intended that all the matter contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Iclaim as my invention:

l. A fuse comprising a generally tubular, electrically insulating casing, terminal means disposed adjacent to each of the opposite ends of said casing, one of said terminal means including an opening at one of the axial ends of said fuse, one or more fusible elements disposed in said casing and connected between said terminal means, a generally tubular, electrically insulating housing disposed in the casing with one end of said housing extending around said opening in said one of said terminal means, a generally cylindrical plunger normally disposed in said housing and including a flange portion intermediate the axial ends of said plunger and an axially outer portion disposed on one side of said flange portion adjacent to the opening in said terminal means, a spring disposed in said housing with one end of said spring bearing against the flange portion of said plunger for biasing said plunger outwardly from said housing, a restraining wire connected between said plunger and the other of said terminal means for normally retaining said plunger within said housing against the biasing force of said spring, stop means disposed around the opening in said terminal means for limiting the outward movement of said plunger, and a resilient gasket member slidably disposed on the outer portion of said plunger between said flange portion of said plunger and said last-mentioned means to be compressed by said spring to cushion the impact of said flange portion of said plunger against said last-mentioned stop means under the influence of said spring and to form a gas seal between the flange portion of said plunger and the inner bore of said housing when said restraining wire ruptures.

2. The combination as claimed in claim 1 wherein said limiting means comprises a generally tubular member disposed to project through the opening in said terminal means axially into the outer end of said housing.

3. The combination as claimed in claim 2 wherein said gasket member comprises an O-ring gasket.

4. The combination as claimed in claim 1 wherein said limiting means comprises a portion of said one of the terminal means which is turned inwardly around the opening in said one of the terminal means. i

5. The combination as claimed in claim 4 wherein said gasket member comprises an O-ring gasket.

6. The combination as claimed in claim I wherein said gasket membercomprises an O-ring gasket.

7. The combination as claimed in claim 1 wherein said gasket member is formed from Buna-N-type synthetic rubber.

8. A fuse comprising a generally tubular, electrically insulating casing, ,a pair of electrically conducting ring members disposed at the opposite ends of said casing, a pair of electrically conducting ferrules secured to the opposite ends of said casing, each of said ferrules including a first end portion which is axially spaced from the ring member at the same end of said casing and a second portion which extends axially over one of said members and over the adjacent end of said casing, each of said ring members including a central opening and a plurality of inwardly projecting portions angularly spaced around said opening to define at least first and second radially spaced groups of circumferentially spaced support surfaces around said opening, the corresponding support surfaces of said ring members being substantially aligned with each other, a plurality of fusible elements extending between and engaging corresponding support surfaces of said ring members, the ends of each fusible member extending over the outer peripheries of said ring members and the ends of said casing to be secured between the respective ferrules and said casing, said casing being substantially filled with an arc quenching material in which said plurality of fusible elements is embedded.

9. The combination as claimed in claim 8 wherein each of said one or more fusible elements is fonned from ribbonshaped conducting material and the supporting surfaces of said ring members are disposed at the outer ends of radially extending recesses of a width adapted to receive each fusible element.

10. The combination as claimed in claim 8 wherein a generally Z-shaped electrically conducting wire member is spaced from the first end portion of one of said ferrules and is disposed to extend diametrically across the opening in said one of said ring members with the transverse outer portions of acent ferrule, and said restraining wire being embedded in said wire member bearing against the second portion of the adjacent ferrule and a spring biased indicating means is mounted on the other ferrule with a restraining wire being normally connected between said generally Z-shapcd wire member and said indicating means, said'generally Z shaped wire member providing an electrically conducting path between said indicating wire and the second portion of the adsaid arc-quenching material.

' a a s a 10- 

1. A fuse comprising a generally tubular, electrically insulating casing, terminal means disposed adjacent to each of the opposite ends of said casing, one of said terminal means including an opening at one of the axial ends of said fuse, one or more fusible elements disposed in said casing and connected between said terminal means, a generally tubular, electrically insulating housing disposed in the casing with one end of said housing extending around said opening in said one of said terminal means, a generally cylindrical plunger normally disposed in said housing and including a flange portion intermediate the axial ends of said plunger and an axially outer portion disposed on one side of said flange portion adjacent to the opening in said terminal means, a spring disposeD in said housing with one end of said spring bearing against the flange portion of said plunger for biasing said plunger outwardly from said housing, a restraining wire connected between said plunger and the other of said terminal means for normally retaining said plunger within said housing against the biasing force of said spring, stop means disposed around the opening in said terminal means for limiting the outward movement of said plunger, and a resilient gasket member slidably disposed on the outer portion of said plunger between said flange portion of said plunger and said lastmentioned means to be compressed by said spring to cushion the impact of said flange portion of said plunger against said lastmentioned stop means under the influence of said spring and to form a gas seal between the flange portion of said plunger and the inner bore of said housing when said restraining wire ruptures.
 2. The combination as claimed in claim 1 wherein said limiting means comprises a generally tubular member disposed to project through the opening in said terminal means axially into the outer end of said housing.
 3. The combination as claimed in claim 2 wherein said gasket member comprises an O-ring gasket.
 4. The combination as claimed in claim 1 wherein said limiting means comprises a portion of said one of the terminal means which is turned inwardly around the opening in said one of the terminal means.
 5. The combination as claimed in claim 4 wherein said gasket member comprises an O-ring gasket.
 6. The combination as claimed in claim 1 wherein said gasket member comprises an O-ring gasket.
 7. The combination as claimed in claim 1 wherein said gasket member is formed from Buna-N-type synthetic rubber.
 8. A fuse comprising a generally tubular, electrically insulating casing, a pair of electrically conducting ring members disposed at the opposite ends of said casing, a pair of electrically conducting ferrules secured to the opposite ends of said casing, each of said ferrules including a first end portion which is axially spaced from the ring member at the same end of said casing and a second portion which extends axially over one of said members and over the adjacent end of said casing, each of said ring members including a central opening and a plurality of inwardly projecting portions angularly spaced around said opening to define at least first and second radially spaced groups of circumferentially spaced support surfaces around said opening, the corresponding support surfaces of said ring members being substantially aligned with each other, a plurality of fusible elements extending between and engaging corresponding support surfaces of said ring members, the ends of each fusible member extending over the outer peripheries of said ring members and the ends of said casing to be secured between the respective ferrules and said casing, said casing being substantially filled with an arc quenching material in which said plurality of fusible elements is embedded.
 9. The combination as claimed in claim 8 wherein each of said one or more fusible elements is formed from ribbon-shaped conducting material and the supporting surfaces of said ring members are disposed at the outer ends of radially extending recesses of a width adapted to receive each fusible element.
 10. The combination as claimed in claim 8 wherein a generally Z-shaped electrically conducting wire member is disposed adjacent to one of said ring members and axially spaced from the first end portion of one of said ferrules and is disposed to extend diametrically across the opening in said one of said ring members with the transverse outer portions of said wire member bearing against the second portion of the adjacent ferrule and a spring biased indicating means is mounted on the other ferrule with a restraining wire being normally connected between said generally Z-shaped wire member and said indicating means, said generally Z-shaped wire member providing an electrically conductinG path between said indicating wire and the second portion of the adjacent ferrule, and said restraining wire being embedded in said arc-quenching material. 